I found a Fenix P3D laying around not getting used, because it takes two CR123’s, and the upkeep is brutal using primaries.
Are there any ways to get this light working with rechargables?
You can order Titanium Innovations Titanium Innovations CR123A 3V Lithium Battery - BatteryJunction.com Just $1.00 each or use an 16650 battery. Vapcell INR16650 2000mAh M20 (Gold) 2020 is the latest.
Is it a first gen light? I believe only the very first few were not compatible with 16340. The spec sheet for the P3D CE model even mentions rechargeables: https://www.fenixtactical.com/manuals/fenix-p3d-ce-flashlight.pdf
I was gifted a hard anodized P3D made in late 2008, last year and did a runtime on turbo with 2 x Imren 16340 cells. Brightness is ok, I think “turbo” is 150 lumens 
When the light was contemporary I had one for my EDC and was blown away by how bright it was. 2008/2009 I wasn’t into rechargeable lithium cells so I fed it probably a couple hundred bucks worth of CR123A cells over the years until I retired it. P3D is a good light, long obsolete but sometimes it’s fun to use old tech 
Aren’t the RCRs 3.7V rechargeables?
Yes, however current is going to be your run time and RCR123a’s only hold about 600 mah. CR123a about 1550 mah. 16650 about 1900 mah.
I do not know if it is a first gen light or not.
It is from 2008, or maybe 2007.
Back then it was jaw-droppingly good. It seemed so bright one never used it on turbo.
Different days for sure!
On the side of the head it does say:
P3D
Premium Q5
As with most of my lights, no matter their age I try to have them in working order.
Even if I’m not the one using them, I want to be able to just hand it to someone and they can use.
It doesn’t feel right to hand them a light that can chug through 123A’s like no tomorrow,
as there is zero chance they’d replace the batteries after using it;
so rechargables it is!
Ha I remember those days well 
If it’s a Q5 model it’s definitely compatible with 16340 and 16650.